Praxis II: Chemistry Exam Prep

Answer: Bromine

The mass number of an element is the number of protons and neutrons per atom. Subtracting the number of neutrons from the mass number gives the number of protons in an atom, or the atomic number. In the context of the question, subtracting 45 from 80 gives an atomic number of 35. From here, it is easy to identify the element as bromine.